Protect your possessions. What I mean by this is to keep your money/credit cards/passports/other personal possessions in your sight. If don’t need to carry these possessions with you, put them in a safe/lock box in your hotel.
Be aware and cognizant of your surroundings. It’s just like what you learned at an early age to look both ways before you cross the street. I would recommend looking left and right and left and right again in The Netherlands, as there are regular cars and cabs and buses, but there are also many MANY bikes. The bikers have their own, clearly marked lanes on some of the roads. This is their space, and, if you want to avoid any mishaps, RESPECT IT!